You're giving the file a relative path and expecting it to magically find the file. Relative paths are resolved relative to where your JVM was started. If you want an absolute path you'll need to do something like:
new File("C:\\Users\\Gabrielle\\Documents\\NetBeansProjects\\CS2 Assignment 5\\src\\index.html");
You're not getting that from the request to your application because the request is URLEncoded.
Of course, the better solution would be to have the file in a reasonable place relative to your application and reference it relatively.